Currently, color images are generated through a wide variety of different systems, such as for example photographically on suitable film or photosensitive paper, or electronically on video tape or other suitable media. When generated, images share a basic characteristic: they are recorded on a continuous tone (hereinafter referred to as "contone") basis. As such, recorded color information at any point in the image is represented by several continuous amplitude values, each of which is oftentimes discretized as eight-bit values ranging from "0" to "255". Very often, a user having an image captured on one medium, such as a photographic print or transparency, will desire to display and/or reproduce that image on other media, such as on a video monitor or on a printed page.
Color reproduction equipment, as it relates to printing images, takes advantage of the principle that the vast majority of colors can be separated into a specific combination of four primary subtractive colors (cyan, yellow, magenta and black--C, Y, M and K) in which the amount of each primary color is set to a predetermined amount. In the case of printed reproductions of an image, use of primary color printing obviates the need to use a differently colored ink for each different color in the image. As such, each image is commonly converted into sets of three or four color separations, in which each separation is essentially a negative (or positive) transparency with an altered tone reproducing characteristic that carries the color information for only one of the primary colors. Separations are subsequently recorded on printing plates for use in a press.
By way of contrast, color reproduction on cathode ray tube displays takes advantage of the principle that the vast majority of colors can be represented by a combination of three primary additive colors (specifically red, green and blue--R, G and B) in which the intensity produced by each primary colored (R, G or B) phosphor is set to a predetermined amount.
Modern offset printing presses do not possess the capability of applying differential amounts of ink to any location in an image being printed. Rather, these presses are only designed to either apply or not apply a single amount of ink to any given location on a page. Therefore, an offset printing press is unable to directly print a contone separation. To successfully circumvent this problem, halftone separations are used instead. An image formed from any single color halftone separation encodes the density information inherent in a color image from amplitude modulated form into a spatial (area) modulated form, in terms of dot size, which is subsequently integrated by the human eye into a desired color. By smoothly changing halftone dot sizes (dot areas), smooth corresponding tone variations will be generated in the reproduced image. Given this, the art has taught for some time that a full color image can be formed by properly overlaying single color halftone reproductions for all of the primary subtractive colors, where each reproduction is formed from a corresponding halftone dot separation that contains dots of appropriate sizes. Clearly, as size and spacing of the dots decrease, an increasing amount of detail can be encoded in a halftone dot pattern and hence in the reproduced image. For that reason, in graphic arts applications, a halftone separation utilizes closely spaced dots to yield a relatively high resolution.
With this in mind, one might first think that printing a color image for graphic arts use should be a fairly simple process. Specifically, a color image could first be converted into corresponding continuous tone separations. Each of these contone separations could then be converted into a corresponding halftone separation. A printing plate could then be manufactured from each halftone separation and subsequently mounted to a printing press. Thereafter, paper or other similar media could be run through the press in such a fashion so as to produce properly registered superimposed halftone images for all the subtractive primary colors thereby generating a full color reproduction of the original image.
In practice, accurately printing a color image is oftentimes a very tedious, problematic and time consuming manual process that requires a substantial level of skill. First, the conventional manual photographic process of converting a contone separation into a halftone separation, this process commonly being referred to as "screening", is a time and resource consuming process in and of itself. Second, various phenomena, each of which disadvantageously degrades an image, often occur in a reproduced halftone color image. Moreover, the complete extent to which each of these phenomena is present in the reproduced image is often known only at a rather late point in the printing process thereby necessitating the use of tedious and time and resource consuming iterative experimentation to adequately eliminate these phenomena.
Traditionally, on-press proofing provided the first point at which a color judgment could be made regarding the quality of the reproduced image. For example, many color differences, such as incompatible and/or objectionable color renditions or Moire patterns, were usually first seen at this point in an imaging process. If such a difference were sufficiently objectionable to a color technician, then usually the entire imaging process would need to be modified and repeated. Doing so generally necessitated a total rework of the separations, production of a new set of printing plates therefrom and generation of a new press proof, with this process being iteratively repeated as many times as necessary to properly remove or sufficiently attenuate the incompatible and/or objectionable color differences.
In an effort to reduce the time required and expense associated with conventional manual photographic based color reproduction processes and particularly the traditional on-press proofing techniques used therewith, the art has turned away from use of on-press proofing in high volume graphic art applications and towards the use of intermediate off-press proofing technologies, such as electro-photographic techniques. In this regard, U.S. Pat. No. 4,708,459 (issued to C. Cowan et al on Nov. 24, 1987, assigned to the present assignee hereof and hereinafter referred to as the U.S. Pat. No. '459 Cowan et al patent) discloses an electro-photographic color proofing system (also referred to herein as a "proofer") with variable tone reproduction characteristics.
While the proofing system described in the U.S. Pat. No. '459 Cowan et al patent provides an excellent quality proof, this system, like all imaging systems, can reproduce colors only within a certain color gamut. Generally speaking, the tone reproduction characteristics of one type of imaging system, or even one type of imaging medium, are not completely coincident with those of a different type of imaging system or medium. In this regard through use of differing colorants (inks used in printing as compared to photographic dyes or colored phosphors on a video monitor) and other physical phenomena related to specific imaging processes, a given color shown on a color artwork, such as on a photograph, or on a press sheet printed on publication stock will often appear differently in a halftoned color proof formed on electro-photographic film and subsequently transferred to paper, the latter having characteristics similar to paper which will be used in a press. Furthermore, a halftone color proofing system, such as that described in the U.S. Pat. No. '459 Cowan et al patent, is generally incapable of producing the exact same color gamut and color response which are available through either the photograph or press sheet. In this regard, the color gamut reproducible in a color halftoned proof will generally not match that associated with a color artwork that appears on a photograph or on a press sheet. In addition and owing to physical differences among different imaging systems, the response of different types of imaging systems to an identical input color will likely be different, e.g. the same red color provided as input to two different imaging systems might likely produce two output colors with somewhat differing red hues.
In view of the inherent tone and color differences between, e.g., the press sheet and the proof thereof, the colors in the proof can not be identically matched to those that appear in the press sheet. Nevertheless, for a proofing system to fully serve its intended purpose, a proof image must accurately predict the image as subsequently printed on a press sheet. However, the tone and color reproduction characteristics of a proofing system rarely coincide with those of an associated press. Therefore, the tone and color reproduction characteristics of the proofing system must be calibrated, to the extent possible, to those of the press. Once calibrated, the proofing system should be able to accurately predict the performance of the press though, in most situations, it will generate a proof image with colors that do not exactly match those in the press sheet.
Unfortunately, calibrating a proofing system tends to consume an inordinate amount of time as well as require a very high level of skill. In this regard, a color technician is required to possess a substantial level of skill and expertise not only to judge color differences between a proof and a press sheet therefor but also to fully appreciate the performance inter-relationships between the colors that appear on the proof and the corresponding ones that will appear on the press sheet. Consequently, the technician not only must recognize a color difference and decide which specific colors to match but also, where the tone and color reproduction characteristics of the proofing system can be varied, determine the proper variations in these characteristics in order to achieve an acceptable match between the proof and the press sheet and then set the proofing system accordingly.
In particular, to calibrate a proofing system to a press, a color technician usually visually examines both a proof and the associated press sheet on a side-by-side basis and then, based upon his own subjective judgment as to what the visually important features of the press sheet are and how they should appear, selects which colors to match. Thereafter, given his knowledge of the response of the proofing system and its color response, he will attempt to initially vary the C, Y, M and K colorant solid area densities and/or dot size (tone reproduction curve) settings to accurately depict one color(s), which, not surprisingly, will also affect other colors, possibly adversely. Based upon the effects that occur with respect to other colors in the proof, the technician will iteratively vary the solid area densities and/or dot size (tone reproduction curve) settings of the colorants, in seriatim, until an acceptable color match is achieved between the press sheet and the proof for the selected colors.
However, a proofing system with variable tone and color reproduction characteristics often presents the technician with an enormous number of different possible combinations of the settings. For example, for the system described in the U.S. Pat. No. '459 Cowan et al patent, the solid area density and dot size can be set for each of the four process colors (C, Y, M and K) at any of 20 different density levels and at any of 15 different dot size settings. In view of the resulting huge number of potential combinations of settings, an experienced color technician often needs to run and separately analyze quite a few successive proofs in order to select a suitable solid area density and halftone dot size setting for each different colorant in order to achieve an acceptable match between the proof and a press sheet and thereby calibrate the proofing system to the press. Moreover, additional time is consumed whenever the technician is forced to resort to trial-and-error experimentation or, in a worst case scenario, guesswork: either merely as a result of iterating through a very large number of possible combinations to discern the performance inter-relationships of the proofing system and/or by incorrectly relying on intuition and initially iterating away from a proper operating condition. An example of the latter situation can occur where the technician, based upon his own intuition, views a proof against a press sheet and decides that the yellow content in the proof needs to be increased. While the technician may decide to initially increase the halftone dot size for the yellow colorant, the proper operating condition may instead involve reducing the halftone dot sizes for all the colorants but reducing the halftone dot size for yellow less than that for each of the other colorants.
Furthermore with certain images, the technician may simply have insufficient skill to quickly determine the proper operating conditions of the proofing system. As such, in certain situations, the technician, given his lack of knowledge or experience, may be unable to determine the best possible color match in the time allotted and thus must settle for one that is often simply acceptable. In view of this, empirical approaches have been developed to aid the technician in quickly locating a limited region of the operating space of the proofing system in which a decent match can be achieved to which the proofing system can be calibrated. One such empirical approach could involve first matching the C, M, Y and K solid area and halftone densities between the press sheet and the proof to the extent realistically possible--though this may generally produce mis-matches in overprint colors, e.g. the reds, greens and blues. Once these primary color matches are achieved, the resulting proof is then visually examined to determine how certain overprint colors appear, e.g. whether the gray tones are the same as on the artwork or are too red. If the latter occurs, then the colorants are appropriately changed, possibly through successive iterative changes, to increase the cyan content or decrease the magenta and yellow content in the proof. Alternatively, the technician could visually examine the reds in the proof. If the reds appear too orange, the colorants could be appropriately changed to decrease the yellow content of the proof or alternatively increase its magenta content. In that regard, it is widely known that an average human vision is acutely sensitive to flesh tones (which specifically contain red hues). Hence, even a subtle difference in coloration may be perceived as transforming an otherwise pleasant image of a human face into one that is quite unnatural and obnoxious. Through such approaches, even a skilled color technician may still need to generate upwards of 12-15 separate proofs in seriatim, typically requiring a full day of work, until he discerns the proper operating condition of the proofing system which is needed to achieve an acceptable color match between the proof and a press sheet therefor and thereby calibrate the proofing system to the press in use.
Through a totally different approach, the technician could quantitatively measure reflection densities of selected portions of the image on both the press sheet and the proof using, for example, a reflection densitometer, and then attempt to set the colorants in a manner that seeks to achieve the densities inherent in the press sheet. Unfortunately, this approach is constrained by the ability of the technician to locate corresponding relatively large uniformly colored areas on both the press sheet and the proof at which the reflection densitometer can be reliably placed to take measurements. If both images contain significant detail, then suitable measurement areas may not exist and thereby preclude such densitometric measurements from being made.
Apart from a reflection densitometer, one device that has recently become available for color measurement and matching is a spectrophotometer, such as the Model SPM 50 spectrophotometer manufactured by Gretag Corporation of Bothell, Wash. This device projects white light onto an image and then separates the spectrum of reflected light from the image through a diffraction grating and measures the intensity of the reflected radiation at a number of different wavelengths. Through this device and its associated software, colorimetric spectral based measurements can be made of any reflection image. Though this device is intended to be used to determine proper halftone dot size in the separations in order to achieve a desired coloration in the reflection image therefrom, conceivably it could be used to characterize (i.e. "model") a proofing system in use and then effectuate a color balance between a press sheet and a proof therefor. Specifically, a set of known test (reference or calibration) separations is provided with the device, and dot area settings for these separations are stored within the device. To characterize the proofing system, a proof is made from the reference separations. Thereafter, spectrophotometric measurements are taken of this particular proof. The resulting measurements, when processed, would yield a model that characterizes the color gamut producible through the proofing system. Thereafter, in order to generate a color match to a press sheet, the device could then be used to take spectrophotometric measurements of the press sheet. Given the characterization of the proofing system and the latter set of measurements, the software will determine appropriate values to use for solid area densities and corresponding halftone dot sizes for each primary colorant in the proofing system in order to generate a proof that should match the press sheet.
Inasmuch as the color gamut reproducible through a proofer does not coincide with that appearing in the press sheet, the software used with this device is constrained, just as the technician is in manually performing a color match, to effectuate a compromise in matching the two gamuts between the press sheet and the proof therefor. In achieving a color match, this software relies on the well-known CIELAB L*,a*,b* color coordinate system and color differences associated therewith. In computing a color match, the software seeks to minimize an overall .DELTA.E value (i.e. the sum of the squares of the CIELAB color difference values) between the two color gamuts and thus obtain a "colorimetric" match.
In this regard, a very small colorimetric difference for some colors will lead to a very large .DELTA.E value; while this will not be true for other colors. Any system, such as the Gretag spectrophotometer, that seeks to minimize an overall colorimetric error between two images produced by systems with differing tone and color reproduction characteristics may well still produce minor color mismatches for some colors that, in various image contexts, would be highly objectionable to a human observer.
In particular, it has been known for some time that human color perception, including mental judgment, exhibits differing sensitivities for different colors. Given this, human observers will be much more acutely aware of what would amount to minor color differences, such as differences in so-called "memory" colors (e.g. greens and flesh tones), in certain pictorial contexts than in others. Accordingly, a color difference that would simply be noticeable, if at all, in some contexts would be highly objectionable in others. For example, people are acutely aware of very small differences in flesh tones. A viewer will likely object to a human face that appears too blue or green, while merely noticing, if at all, and certainly not objecting to a tablecloth or blanket that exhibited the same variation. Thus, an effective color balance needs to account for the preferences inherent in human color perception. Specifically, if a press sheet is compared side-by-side to an accurate proof thereof, a viewer should reach the conclusion that the proof in effect has a good appearance, i.e. flesh tones appear as they should as well as do other colors given the context of the image thereon. In this instance, the relative coloration throughout the proof is pleasing even though the specific hues in the proof will not necessarily identically match those in the press sheet. Such a visually pleasing match between, for example, a proof and a press sheet will hereinafter be referred to as an "appearance match".
Any system that attempts to provide a uniform "colorimetric" match across all colors totally ignores the innate preferences inherent in human color interpretation. Consequently, the resulting proof, based solely on a colorimetric match to the press sheet, is likely to contain minor color differences, that depending upon the context of the particular image, can be highly objectionable to a viewer. Consequently, the proof would not be a visually appealing representation of the press sheet. In these instances, a colorimetric based approach to color matching will clearly yield an unsatisfactory match that simply can not be used to calibrate a proofing system. When this occurs, a color technician would likely revert back to a manual approach to locate what he subjectively perceives to be an appearance match between the press sheet and the proof--but will be forced to accept a rather high cost in time and material to do so.
As one can now appreciate, thus far the art has simply failed to provide a relatively fast systematic technique for objectively and automatically achieving a satisfactory appearance match between one color image, such as a press sheet, and another image, such as a proof therefor, for nearly all images, and particularly for use in calibrating one imaging system, such as a proofer, to another, such as a press.
Furthermore, proofs are typically printed, particularly through the proofing system described in the U.S. Pat. No. '459 Cowan et al patent, with so-called "run bars" that appear alongside the proof image. Each run bar provides a pre-defined sequence of color test patches composed of various combinations of primary colors, both as solids and halftones, as well as other diagnostic targets. These bars are intended to provide test areas for taking densitometric measurements of areas of uniform colors. For example, a typical run bar will contain, among others, separate solid and 50% input color patches for the K, C, M and Y primary colors; separate solid color patches for the red (M and Y overprint), green (C and Y overprint), blue (C and M overprint) and three-color overprints (C, M and Y); and a 50% input color patch for a three-color overprint. Inasmuch as these bars exist alongside the proof and are formed by the proofing process using the same colorants as in the proof itself, these bars identically represent the result of the proofing system tone and color reproduction characteristics in the proof. Given the existence of the run bars in proofs, particularly those produced through the proofing system described in the U.S. Pat. No. '459 Cowan patent, it would be highly desirable to utilize these bars in some fashion to effectuate a color match between the proof and a press sheet. Similar run bars exist on a press sheet.
Therefore, a need currently exists in the art for a technique, specifically though not exclusively intended for inclusion in a proofing system, that can be used, particularly in conjunction with run bars or other simple test objects, to quickly, objectively and automatically provide an appearance match between an image produced by one imaging system, such as a proof, and another image, such as a press sheet, produced therefor but from a different imaging system. While the colorations in such a match will rarely be identical due to differences in reproducible color gamut and color response between these two systems, the match attained through this technique should nevertheless result in, for example, a proof that is, in substantially all instances, a visually accurate and appealing representation, i.e. an appearance match, of a press sheet. Moreover, by automatically providing a match through objective criteria, such a technique should significantly reduce the trial-and-error effort and the degree of skill required of a user, as well as the associated time and cost, needed to achieve such a match and thereby calibrate a proofing system to a press.